Generators or dynamos used for bicycles are well known. Several different variants of generators exist, ranging from very simple dynamos where a multiple pole permanent magnet is rotated relative to a single coil via a small wheel in frictional engagement with the rim of the tire of the bicycle. Other types of generators are based on magnets which are attached directly to the wheel or tire and which induces a current in a coil which is attached to the frame of the bicycle. Yet another type of generator is based on simple multiple pole generators which are arranged in the hub of the front wheel.
WO 2009015910 discloses a generator for a bicycle. The generator has a driving magnet fixed to a wheel of the bicycle and an induction structure which is fixed to the frame of the bicycle. The induction structure comprises an induction magnet which is movably fixed to a coil. A fixture allows fixing of the magnets at locations where they, during normal operation of the bicycle, repeatedly moves towards and away from each other so that the driving magnet moves the induction magnet relative to the coil. To provide a generator which can potentially deliver a uniform output which is less dependent on a very specific installation of the generator, on the bicycle and which may therefore be easy to install, the generator further comprises a resetting magnet which provides positioning of the induction magnet relative to the coil when the driving magnet moves away from the induction magnet.
Whereas WO2009015910 shows improvements in comparison with known generators for bi-cycles, the use of a resetting magnet has some problems. For example, the strength of the resetting magnet has to be chosen carefully. That is, if it is too strong it may hinder efficient movement of the induction magnet, and if it is too weak it may not be able to position the induction magnet. Furthermore, since the air gap between the driving magnet and the induction magnet may depend on a particular generator installation so that the magnetic force provided by a driving magnet is not accurately known, the correct choice of the strength of the reset magnet may be further complicated.
Furthermore, the resetting magnet may disturb the magnetic flux generated by the induction magnet and thereby reduce the amplitude of current induced in the coil by the induction magnet.
Due to these problems the inventor of the present invention has appreciated that an improved induction generator is of benefit, and has in consequence devised the present invention.